Molecular emission from the diffuse gas around ζ Ophiuchi

Wehavemappedλ18cmOH andλ9cmCH emission from the diffuse gas around ζ Oph, and find no trace of emission from the +4.5 km s−1 gas whose OH line was previously believed to represent pre-shock gas. The pattern of OH emission is strikingly dissimilar to those of CO, HCO+ and CH in that the strong +1 km s−1 component seen to the South is largely absent in OH. CH more closely resembles CO, peaking on either side of the star, and comparison of CO and CH shows that the CO line ‘turns on’ over a very narrow range of CH line strength. Alternatively, OH emission actually weakens as CO brightens in the +0.4 km s−1 line, but this may be only an excitation effect, and not the result of a declining OH abundance. Wemappedλ3mmHCO+ emission over the inner portion of the gas distribution occulting ζ Oph, finding that its behaviour roughly parallels that of CO. We searched for emission from HCN, CS (J=2-1), CN, and C2H at the CO and HCO+ emission peak 30′ South of the star. Of these, only HCN was found and it is quite weak compared to HCO+ (T∗ r ≈ 0.03 K vs. 0.09 K). Emission from the other mm-wave species is absent in spectra with rms noise ∆T∗ r ≈ 0.01 K; CS J=2-1 emission was sought also unsuccessfully toward the star. The column densities inferred from these intensities are uncertain owing to unknown physical conditions along the line of sight, chiefly the kinetic temperature, but they are not necessarilymuch larger thanwould be found toward the star. Relative abundances of the high-dipole moment species are better determined; N(HCN)/N(HCO+)≈ 1, N(CS)/N(HCO+) < 1.